A naval radar system searches space under control of command and decision processing by means of a plurality of sequential directional beams which may be pointed in a given direction. Command and Decision determines the acquisition face to be searched. The radar beam is directed to each angle so as to cover the entire search face. This type of searching is subject to time constraints, as the beam must dwell at the current beam angle for a sufficient time for the transmitted radar signals or pulses to travel to the target, which might be at the maximum allowable range, and for the reflection to return to the radar.
The naval radar can operate in a volume search mode. When information becomes available from another source, such as a cooperating radar, about the possible presence of a target in a nominal given direction or location, it may be desired to examine a volume about the nominal given direction in an attempt to acquire the target. This is termed a “cued” search. If the selected volume is too large, the search may time-out before completion of the search, and if too small, may not find the relevant target(s).
U.S. patent application Ser. No. 12/208,588 generally describes a method for searching an angular region of the radar acquisition search volume (search volume) about a given cued direction and with a given maximum search range. The radar search of the designated volume is performed with sequentially generated radar beams having defined beamwidths. The method comprises the steps of acquiring the nominal track position and velocity (cue information) and time, and error information describing the uncertainty in the cue information. This error information may be presented together with the cue information. From the error information, the azimuth and elevation extent (the acquisition or search face) of the search volume about the cue direction is determined.
In the scenario 10 of FIG. 1, a line 12 defines the horizon. A ship (ownship) 14 carries a radar system, portions of which are illustrated as 16, and a computer processor, illustrated as a block 14c. A target 20 is at a distance from ownship 14, and is observed along a line-of-sight represented by a dot-dash line 24 by means of sensors (not separately illustrated) mounted on a ship 22. Ship 22 obtains information about the location of target 20. Since ownship 14 (and possibly other ships and assets associated with ship 22) may not be aware of the presence of target 20, ship 22 transmits coordinates of the target to other assets and in particular to ownship 14. This transmission may be made by any communication path, such as, for example, the uplink 30U and downlink 30D associated with a communication spacecraft 32. Computer processing aboard ship 22 may evaluate the quality of the target, and transmit target quality or error information together with the target coordinates.
In FIG. 2, ownship 14 includes a communication antenna 212 which is intended for communication with other assets, including the communication represented in FIG. 1 by path 30D. This communication, including information relating to the location of target 20 and the errors associated with the location, is coupled to a command and decision function, illustrated as a block 216. Block 216 of ownship 14 digitally processes the target location information from ship 22 of FIG. 1 in computer (processor) 14c, and from this location information determines the target azimuth and elevation angle relative to ownship 14, and the azimuth and elevation extent of the search or acquisition face required to acquire the target with its own sensors.
The target azimuth and elevation relative to ownship 14 of FIG. 2, and the azimuth and elevation extent of the acquisition face (see FIG. 3A) required by the errors in target azimuth and elevation, are transmitted from Command and Decision block 216 of FIG. 2 to an ownship radar beam control illustrated as a block 218, which may also be part of computer 14c, or which may be a separate computer. Radar beam control 218 commands the generation of transmit and receive beams by radar array antenna face 220. These beams are “pencil” or narrow beams, as known in the art. A representative pencil beam is illustrated as 222. Radar beam control 218 may also receive commands from other functional modes, such as wide-area search modes, illustrated together as a block 224.
The radar beam controller 218 of FIG. 2, together with the antenna face 220, produces sequential pencil beams in directions which, in conjunction with other pencil beams, suitably search the volume of space defined by the combination of an acquisition face in conjunction with the desired range.
FIG. 3A is a representation of a search or acquisition face 310 defined by sequential beam generation by the radar sensor 16 of FIG. 2. The azimuth and elevation directions are indicated by arrows. In FIG. 3A, the nominal target azimuth and elevation, as specified by the target azimuth and elevation angle relative to ownship 14 generated by block 216, appears as a + symbol at the center of the acquisition face 310.
A “cross-section” of each pencil beam is illustrated as a circle. Representative circles are designated by the number 320. The pencil beams are directed so that the beams overlap at a given power level. This overlap is indicated in FIG. 3A by the overlap of the circles. Those skilled in the art will understand that the “magnitude” of the overlap depends upon the “beamwidth” of the beams, the relative placement of the beam centers by the radar, and the attenuation or “signal” level at the overlap. Also in FIG. 3A, the overlapping beams provide coverage of a region defined by a rectangular outline 312. The azimuth “extent” of the coverage region is defined by the arrow designated Aext, extending in the horizontal direction from a vertical centerline 314 to the outline 312. The elevation “extent” of the coverage region is defined by the arrow designated Eext, extending in the elevation or vertical direction from a horizontal centerline 316 to the outline 312.
The relationship of the search or acquisition face to the overall search volume associated with the face is illustrated in FIG. 3B. In FIG. 3B, the acquisition search face is designated as 310, and the range provides a third dimension which defines the search volume 350.
It is critical that the calculated size of the search volume describe provided covariance/errors as closely as possible because the radar can only use minimum and maximum range, nominal azimuth and elevation, and azimuth and elevation extents.
Existing methods for determining search volumes produce inaccurate descriptors of the volume. One of these existing methods uses projection for estimating the search volume, but does not incorporate the significant effects of orientation of the error or covariance ellipsoid. Cross-range extents of the search volume to either side of center contain unnecessary error by being displaced in one or the other direction.
If the descriptors, i.e., the estimated elevation, azimuth, and range extents, of the search volume are inaccurate, the radar system may waste resources searching where it is unlikely to find a target, when such resources could have been allocated for other activities. Further, inaccurate descriptors may cause the radar system not to search where the target is likely to be. In addition, predetermined requirements may not be met if the minimum probability of containing a target cannot be estimated.
Accordingly, an improved method and system is desired for determining or estimating the elevation, azimuth, and range extents of search volume.